In a transmission apparatus of a digital wireless communication system and a broadcasting system, a directional coupler is used for combining process or distributing process of a transmission signal. As broadening a bandwidth of the transmission apparatus is requested in recent years, this directional coupler is strongly desired to have a wideband and good characteristic. For this reason, when the directional coupler is manufactured, a structure of multi-stage coupling is usually used.
In the directional coupler, it may be necessary to change a degree of coupling between each transmission line. In this case, a design change of a circuit pattern is obliged and it may be necessary to exchange the whole substrate which constitutes the directional coupler.
As conventional technology related to this kind, there is a radio frequency filter indicated in JP, PH06-97766A. In this high pass filter, two signal lines are formed on mutually different planes and a substrate is sandwiched between both lines. A degree of coupling is adjusted by choosing thickness of the substrate sandwiched.
An example of a directional coupler which has a multi-stage coupling part considered conventionally is shown in FIG. 12. Directional coupler 300 has coupling parts 302, 304 and 306. Coupling parts 302, 304 and 306 are formed by transmission line 310 formed on one surface of substrate 308 and transmission line 320 formed on another surface. In coupling parts 302 and 306 with shallow degree of coupling, transmission lines 310, 320 are coupled between their edges. In coupling part 304 whose degree of coupling is comparatively deep, transmission line 310 counters transmission line 320 via substrate 308, and transmission line 310 and transmission line 320 are coupled between the surfaces. In this directional coupler, substrate thickness tolerance will influence the degree of coupling greatly depending on the thickness of substrate 308 to be used. And in order to adjust the degree of coupling, it is required to exchange substrate 308 whole. Terminals 330 and 340 are connected with ends of transmission line 320 via through holes 332 and 342.
In the directional coupler according to the conventional technology, when the degree of coupling is changed, it is obliged to exchange the whole substrate with other substrate and a cost increases sharply.